This invention relates to switched capacitor N-path filters and to an improved method of synthesizing switched capacitor N-path filters.
A monolithic N-path filter implemented with MOS sampled data techniques is described in the article, "A Switched Capacitor N-Path Filter" by D. J. Allstot and K. S. Tan, IEEE International Symposium on Circuits and Systems, May 1980, pp. 313-316, which is incorporated herein by reference. The idea there is to develop a switched capacitor active-ladder equivalent of a prototype ladder filter with the energy storage or integrating capacitor of each integrator being replaced by N identical commutating capacitors and MOSFET switches which share a common active element. Although this technique gives additional freedom in designing filters with transfer functions having complex poles and zeros and provides fully integrated N-path filters, it has been found that the characteristics of the resultant network only approximates those of the corresponding prototype ladder filter, i.e., the ripple and loss in the prototype and resultant filters may not be the same magnitudes nor that predicted by N-path filter theory.
An object of this invention is the provision of an improved switched capacitor N-path filter.